1. Field of the Invention
This invention relates to microwave bandpass filters, and more particularly, to a filter design which allows further substantial miniaturization, and to an improved method of tuning and operation at cryogenic temperatures.
2. Description of the Prior Art
The use of dielectric resonators in microwave filters results in a significant reduction in size and mass while maintaining a performance comparable to that of waveguide filters without dielectric resonators.
A typical dielectric resonator filter consists of a ceramic resonator disc mounted in a particular way inside a metal cavity. In addition to miniaturization, loss performance, as well as thermal and mechanical stability are also important design objectives for dielectric resonator filters. A number of specific refinements can be incorporated in furtherance of these goals.
For instance, in dielectric resonator filters the size of the cavity can be substantially reduced by mounting the dielectric resonator along a base wall of the cavity rather than mounting the resonator in a center of the cavity. This eliminates the need for a centering stem-type mounting, and it allows a reduction in the size of the microwave cavity. See, U.S. Pat. No. 4,423,397 issued to Nishikawa, et al. However, it is difficult to attach the dielectric resonator to the base wall in such a way that proper electrical contact is ensured. Conductive glues and the like can result in a change in frequency of the filter, thereby reducing the Q (i.e. quality factor). Moreover, this type of mounting is prone to the thermal expansion caused by wide temperature variations, and to the mechanical vibrations that must be endured when the filter is used in space applications.
Multiple mode filters also can provide further miniaturization over single mode filters. For instance, single, dual and triple mode dielectric resonator waveguide filters are known (See U.S. Pat. No. 4,142,164 by Nishikawa, et al., issued Feb. 27th, 1979; U.S. Pat. No. 4,028,652 by Wakino, et al. issued Jun. 7th, 1977; Paper by Guillon, et al. entitled "Dielectric Resonator Dual-Mode Filters", Electronics Letters, Vol. 16, pages 646 to 647, Aug. 14th, 1980; U.S. Pat. No. 4,675,630 by Tang, et al. issued Jun. 23rd, 1987; U.S. Pat. No. 4,652,843 by Tang, et al. issued Mar. 24th, 1987; and U.S. Pat. No. 5,083,102 by Zaki.).
The use of superconductors is a more recent advance which holds good potential. For example, a hybrid dielectric resonator high temperature superconductor filter is known which utilizes a plurality of resonators in a cavity where each resonator is spaced from a conductive wall of the cavity by a superconductive layer. The superconductive layer is capable of superconducting at temperatures as high as about 77.degree. K. Existing super-conductive filters cannot produce repeatable results when these filters are tuned at cryogenic temperatures, then allowed to return to room temperature and subsequently return to cryogenic temperatures. As a result, a heat exchanger is necessary to maintain the filter housings at or below the critical temperature of the superconductor after the filters have been tuned. Any further miniaturization gained by the use of superconductors is undermined by the need to employ a bulky heat exchanger or like refrigerant.
Finally, U.S. Pat. No. 4,881,051 by W. C. Tang, et al. issued Nov. 14th, 1989 describes a dielectric image-resonator multiplexer. The use of image resonators, as disclosed in the Tang '051 patent, allows smaller sectional resonator elements with some degradation in loss performance.
It would be greatly advantageous to improve the miniaturization and loss performance of a dielectric resonator filter by incorporating superconductive materials and image resonators in a simplified design, and to improve the thermal and mechanical stability of the filter by using mounting blocks.